The compressor section of a turbine engine can comprise several rows of airfoils, alternating between rows of rotating airfoils and rows of stationary airfoils. Each row of rotating airfoils comprises a plurality of airfoils held on a disk which is, in turn, attached to a rotor. Thus, the attached airfoils rotate along with the rotor. Each row of stationary airfoils comprises a plurality of stationary airfoils affixed at one end to the inner periphery of a ring or other airfoil carrier such as a diaphragm pack. The stationary airfoils extend radially inward from the airfoil carrier, terminating at an inner shroud.
The general operation of a compressor is well known in the art—the rotating airfoils force a fluid axially through the compressor while the stationary airfoils compress the fluid. As a fluid passes axially through the compressor, the pressure of the fluid increases. For instance, the fluid pressure on the downstream side of a row of stationary airfoils is greater than the pressure on the upstream side of that same row. The compressed fluid will naturally seek the path of least resistance, that is, the lower pressure upstream side of the stationary airfoil, which can result in reverse axial flow through the compressor, thereby degrading performance. To impede such reverse flow, prior compressors incorporated seal strips extending from the inner shroud of the stationary airfoils, thereby reducing the gap between the inner shroud of the stationary airfoils and the nearby rotating airfoils and/or disk.
While helpful in reducing reverse flow, use of the seals can create another concern in that the seals must be prevented from rubbing against the rotating disk and/or airfoils. Seal rubbing during off design conditions has been known to cause the seals to liberate, wrap around the neighboring stationary airfoils, and ultimately cut through the stationary airfoils, resulting in extended outages and hardware replacement. Thus, it is critical to maintain a clearance between the seals and the rotating airfoils at all times.
In the past, the problem of seal clearances has been approached by providing large seal clearances during non-standard engine conditions where the seal clearances would otherwise be expected to be the smallest because of thermal inequalities and other factors. Examples of such non-standard operating conditions include shut down, hot restart, spin cool, etc., all of which occur when the engine is operating at less than about 3600 rpm. However, because the minimum seal clearances are sized for these off design conditions, the seal clearances become overly large when the engine achieves full speed (i.e. normal operation). Consequently, the compressor/engine experiences measurable performance decreases in power and efficiency due to clearance leakage at normal operation.
Thus, there is a need for a compressor system that not only allows for larger compressor seal clearances as the engine passes through non-standard operating conditions, but also minimizes clearances during normal engine operation, thereby increasing efficiency of the compressor.